Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids. Ethylene carbonate is typically used as a solvent.
Monoethylene glycol can be commercially prepared from ethylene oxide by various known methods. These methods, although varied, all generally involve a two-stage reaction system, wherein ethylene is first converted to ethylene oxide, which is then converted to ethylene glycol. In most industrial-scale glycol production operations, the process for the production and recovery of ethylene oxide is integrated with the process for the production of ethylene glycol to maximize energy utilization and reduce costs.
In the first stage, ethylene oxide is typically produced by reacting ethylene with air or elemental oxygen in the presence of a suitable catalyst, such as a silver-based epoxidation catalyst, and often in the presence of organic moderators, such as organic halides, in an epoxidation reactor. (see Kirk Othmer's Encyclopedia of Chemical Technology, 4th edition, Vol. 9, pages 923-940). This reaction generally occurs at pressures of 10-30 bar and temperatures of 200-300° C., and produces an epoxidation reaction product comprising ethylene oxide, unreacted reactants (such as ethylene and oxygen), various impurities (such as aldehyde impurities, including formaldehyde and acetaldehyde) and optionally other gases and/or by-products (such as nitrogen, argon, methane, ethane, water and/or carbon dioxide).
In the second stage, ethylene oxide is converted to ethylene glycol by one of several methods. In one well known method, the epoxidation reaction product from the epoxidation reactor is supplied to an ethylene oxide absorber, along with a recirculating absorbent solution, typically referred to as “lean absorbent”, to absorb the ethylene oxide from the epoxidation reaction product. The ethylene oxide absorber produces an aqueous product stream comprising ethylene oxide, commonly referred to as “fat absorbent”, which is then supplied to an ethylene oxide stripper, wherein steam is usually introduced counter-currently to separate the ethylene oxide as a vapor stream. The separated ethylene oxide is withdrawn at or near the top of the ethylene oxide stripper, as a more concentrated aqueous ethylene oxide stream, while an aqueous stream withdrawn from the ethylene oxide stripper as bottoms is typically recirculated to the ethylene oxide absorber for reuse as lean absorbent. The aqueous ethylene oxide stream withdrawn from the ethylene oxide stripper is then further reacted to provide ethylene glycol, either by direct hydrolysis in a hydrolysis reactor (i.e., by thermally reacting ethylene oxide with a large excess of water) or alternatively, by reacting the ethylene oxide with carbon dioxide in a carboxylation reactor in the presence of a carboxylation catalyst to produce ethylene carbonate. The ethylene carbonate may then be supplied, along with water, to a hydrolysis reactor and subjected to hydrolysis in the presence of a hydrolysis catalyst to provide ethylene glycol. Direct hydrolysis of ethylene oxide typically produces a glycol product of approximately 90-92 wt. % monoethylene glycol (MEG) (with the remainder being predominately diethylene glycol (DEG), some triethylene glycol (TEG), and a small amount of higher homologues), whereas the reaction via the ethylene carbonate intermediary typically produces a glycol product in excess of 99 wt. % MEG.
Efforts have been made to simplify the process for obtaining ethylene glycol from ethylene oxide, reducing the equipment that is required and reducing the energy consumption. For example, GB 2107712 describes a process for preparing monoethylene glycol wherein the gases from the epoxidation reactor are supplied directly to a reactor wherein ethylene oxide is converted to ethylene carbonate or to a mixture of ethylene glycol and ethylene carbonate.
Similarly, EP 0776890 describes a process wherein the gases from the epoxidation reactor are supplied to an ethylene oxide absorber, wherein the absorbing solution mainly contains ethylene carbonate and ethylene glycol. The ethylene oxide in the absorbing solution is supplied to a carboxylation reactor and allowed to react with carbon dioxide in the presence of a carboxylation catalyst, such as an iodide-containing carboxylation catalyst. The ethylene carbonate in the absorbing solution is subsequently supplied, with the addition of water, to a hydrolysis reactor and subjected to hydrolysis in the presence of a hydrolysis catalyst, such as an alkali metal hydroxide.
EP 2178815 describes a reactive absorption process for preparing monoethylene glycol, wherein the gases from the epoxidation reactor are supplied to an absorber and the ethylene oxide is contacted with lean absorbent comprising at least 20 wt. % water in the presence of one or more catalysts that promote carboxylation and hydrolysis and the majority of the ethylene oxide is converted to ethylene carbonate or ethylene glycol in the absorber.
In each of these cases, a recycle gas stream containing gases that are not absorbed by the recirculating absorbent stream will be produced from the ethylene oxide absorber. Typically, at least a portion of this recycle gas stream is treated in a carbon dioxide absorption column and then recombined with any portion of the recycle gas stream bypassing the carbon dioxide absorption column. The combined gases are then recycled to the epoxidation reactor.
However, it has been found that in those processes where the carboxylation reaction is performed in the ethylene oxide absorber using an iodide-containing carboxylation catalyst, decomposition materials and side products may be present in the recycle gas stream and/or in the fat absorbent stream. Examples of such decomposition materials and side products include gaseous iodide-containing impurities, such as alkyl iodides (e.g., methyl iodide, ethyl iodide, etc.) and vinyl iodide.
The silver-based epoxidation catalysts typically used in an epoxidation reactor are susceptible to catalyst poisoning, in particular, poisoning by gaseous iodide-containing impurities, such as alkyl iodides and vinyl iodide. Catalyst poisoning impacts the epoxidation catalyst performance, in particular the selectivity and/or the activity, and shortens the length of time the epoxidation catalyst can remain in the epoxidation reactor before it becomes necessary to exchange the catalyst with fresh catalyst. Accordingly, it is desirable to remove such catalyst poisons as much as is practicable from the recycle gas stream before it comes into contact with the epoxidation catalyst. For example, the use of a purification zone or a guard bed upstream of an epoxidation reactor is disclosed in EP 2285795, EP 2279182 and EP 2155375.
The present inventors have found that the sensitivity of epoxidation catalysts to certain gaseous iodide-containing impurities, particularly alkyl iodides and vinyl iodide, is even greater than previously expected. The present inventors have, therefore, sought to provide improved guard bed systems and improved processes to remove certain gaseous iodide-containing impurities from a recycle gas stream in the manufacture of ethylene oxide, ethylene carbonate and/or ethylene glycol.